The ZFS port to NetBSD is half-done, or maybe more than half. Finish
it and get it really running.

Now, OpenZFS is the main location of ZFS in the Free Software community, and supports FreeBSD, Illumos, Linux and Mac. Probably the right approach is to get the most recent OpenZFS working with NetBSD. This could either be via pkgsrc or via importing and reachover makefiles. The hard part is probably in the ZFS/NetBSD kernel interface bits.